1. Field of the Invention
This invention relates to industrial high relative viscosity (RV) filaments, such as, for use in papermaking machine felts, apparatus and processes for solid phase polymerizing polyamide flake suitable for use in making the filaments, and processes for making the filaments.
2. Description of Related Art
Industrial polyamide filaments are used in, among other things, tire cords, airbags, netting, ropes, conveyor belt cloth, felts, filters, fishing lines, and industrial cloth and tarps. When used as staple fibers for papermaking machine felts, the fibers must have generally good resistance to chemicals and generally good wear resistance (e.g., resistance to abrasion, impact and flex fatigue). Such felts are often exposed to oxidizing aqueous solutions which can seriously shorten the service life of the felt.
Stabilizers are often added to polyamides for the purpose of increasing chemical resistance. The amount of stabilizer which can be introduced is limited, however, due to excess foaming that occurs during polymerization when stabilizers are added to autoclaves or continuous polymerizers (CPs).
It is also desirable to spin filaments which have a high RV to improve resistance to chemicals and to wear from abrasion, impact and flexing. However, in the past, when the polyamide supply for such filaments is polyamide flake, it was often difficult, if not impossible, to obtain the desired high RV while maintaining polymer quality, e.g., low level of cross linking and/or branching.
One way to increase the RV is to increase the amount of catalyst during polymerization in an autoclave, continuous polymerizer (CP), or elsewhere in the process, but this causes process and/or product problems. Difficulties, for instance, similar to those encountered with stabilizers can occur when catalysts are added in high quantities. Further, high quantities of catalysts in the autoclave can cause severe injection port pluggage and complications to injection timings during autoclave cycles. High quantities of catalysts injected into CPs place stringent demands on equipment capability because of high levels of water loading.
In U.S. Pat. No. 5,236,652, Kidder discloses such a process for making polyamide fibers for use as staple for papermaking machine felt. This process comprises (i) melt-blending polyamide flake with a polyamide additive concentrate which is made of a polyamide flake and an additive selected from the group of stabilizers, catalysts and mixtures thereof, and (ii) extruding the melt-blended mixture from a spinneret to form the higher RV fibers. Processes that add catalyst concentrate to polyamide flake, like the Kidder process, require special feed apparatus for metering the concentrate to the flake which significantly increases the expense of operating such a process. Further, adding high concentrations of catalyst to the polyamide often results in process and/or product control difficulties. Cross linking and/or branching of the fiber, and more susceptibility to chemical attack are liabilities of using high catalyst levels in polyamides.
Another way to increase the RV is through solid phase polymerization (SPP) of the polymer. In U.S. Pat. No. 5,234,644, Schutze et al. disclose a post spin SPP process for making high RV polyamide fibers for use in paper machinery webs. In this case, in contrast to prior staple fiber manufacturing processes, the post spin SPP process requires an added step after spinning the fibers with special processing equipment to increase the RV of the fibers. This special equipment adds a significant cost to the producer and the added post spinning step takes additional time to make the fibers. Furthermore, uniform fiber property control is more difficult when the post spinning SPP step is performed in a batch mode.
Thus, there is a long felt need for filaments with higher RV polyamide than previously made, and apparatus and processes for making the filaments for industrial uses, such as, in making papermaking machine felts, without process and product problems, such as those described above.
These and other objects of the invention will be clear from the following description.
The invention relates to a filament for use in papermaking machine felts, comprising:
a synthetic melt spun polyamide polymer;
a formic acid relative viscosity of at least about 140;
a denier of about 2 to about 80 (a decitex of about 2.2 to about 89);
a tenacity of about 4.5 grams/denier to about 7.0 grams/denier (about 4.0 cN/dtex to about 6.2 cN/dtex), and the percent retained tenacity
(i) is greater than or equal to about 50% when immersed for 72 hours at 80xc2x0 C. in an aqueous solution of 1000 ppm of NaOCl,
(ii) is greater than or equal to about 50% when immersed for 72 hours at 80xc2x0 C. in an aqueous solution of 3% hydrogen peroxide, or
(iii) is greater than or equal to about 75% when heated at 130xc2x0 C. for 72 hours.
The invention is further related to an apparatus for solid phase polymerizing polymer flake having a polyamidation catalyst dispersed within the flake and a formic acid relative viscosity of about 40 to about 60 by contacting the flake with substantially oxygen free inert gas, comprising:
a solid phase polymerization assembly for increasing the relative viscosity of the flake, the assembly having:
a vessel with a flake inlet for receiving the flake, a flake outlet for removing the flake after being solid phase polymerized, a gas inlet for receiving the gas, and a gas outlet for discharging the gas; and
a gas system for circulating the gas through interstices between the flake in the vessel, the gas system having:
a filter for separating and removing dust and/or polymer fines from the gas,
a gas blower for circulating the gas,
a heater for heating the gas, and
a first conduit connecting, in series and in turn, the gas outlet, the filter, the blower, the heater, and the gas inlet; and
a serially connected dual desiccant bed regenerative drying system connected in parallel with the first conduit between the blower and the gas inlet, the drying system for lowering the dew point temperature of at least a portion of the circulating gas such that the dew point temperature of the gas at the gas inlet is no more than about 20xc2x0 C.,
whereby solid state polymerization of the flake occurs increasing its formic acid relative viscosity while the gas is circulated through interstices between, thereby contacting, the flake in the vessel at a temperature of about 120xc2x0 C. to about 200xc2x0 C. for about 4 hours to about 24 hours, after which flake having a formic acid relative viscosity of at least about 90 can be removed from the flake outlet.
The invention is also directed to a process for solid phase polymerizing polymer flake having a polyamidation catalyst dispersed within the flake and a formic acid relative viscosity of about 40 to about 60 utilizing substantially oxygen free inert gas, comprising:
feeding the flake into a solid phase polymerization vessel;
separating and removing dust and/or polymer fines from the gas;
drying at least a portion of the gas with a serially connected dual desiccant bed regenerative drying system such that the gas entering the vessel has a dew point of no more than about 20xc2x0 C.;
heating the gas to a temperature of about 120xc2x0 C. to about 200xc2x0 C.;
circulating the filtered, dried, heated gas through interstices between the flake in the vessel for about 4 to about 24 hours; and
removing the flake having a formic acid relative viscosity of at least about 90.
The invention is further directed to a process for melt phase polymerization of polymer for making filaments for use in making staple fibers for papermaking machine felts, comprising:
feeding polymer flake at a temperature of about 120xc2x0 C. to about 180xc2x0 C., into a non vented melt-extruder, the flake comprising:
a synthetic melt spinnable polyamide polymer,
a formic acid relative viscosity of about 90 to about 120, and
a polyamidation catalyst dispersed within the flake;
melting the flake in the melt-extruder and extruding molten polymer from an outlet of the melt-extruder to a transfer line wherein the temperature of the molten polymer in the transfer line within about 5 feet (2.4 m) of the outlet of the melt-extruder is about 290xc2x0 C. to about 300xc2x0 C.;
conveying the molten polymer through the transfer line to at least a spinneret of at least a spinning machine such that the temperature in the transfer line within 5 feet (2.4 m) of the at least a spinneret is about 292xc2x0 C. to about 305xc2x0 C., with a residence time in the melt-extruder and the transfer line of about 3 to about 15 minutes; and
spinning the molten polymer through the at least a spinneret forming a plurality of the filaments having a formic acid relative viscosity of at least about 140.